purple willow
Refers to the plant Salix purpurea. This literal sense is rare in everyday conversation; the word is far more common in the metaphorical expression 蒲柳の質.
The purple willow is a type of willow that grows near water.
delicate constitution; infirmity
Almost always used in the set phrase 蒲柳の質 (ほりゅうのしつ), meaning a weak or frail body. The standalone noun use is very rare.
He has a delicate constitution and easily falls ill.
delicate health; fragile constitution
From Chinese, literally 'cattail willow'. The metaphorical sense comes from the willow's slender, easily broken branches, symbolizing a weak constitution.